London has been battered by 50mph winds that have felled trees and caused travel chaos. Powerful gusts swept across the capital as the Met Office issued a yellow "be aware" weather alert for most of the country.

London has already developed its own mutated strain of swine flu, scientists warned today.

They say the sheer number of infections in the capital has led to the virus mutating into a unique form.

According to the Royal College of GPs, about 12,500 new infections are being reported to London GPs each week.

Wendy Barclay, professor of influenza virology at Imperial College, warned that authorities needed to keep a close eye on London's infections as the virus could mutate into a faster-spreading or more harmful version.

"It is very important to keep a careful look for clusters of severe cases that might indicate that a mutated virus has arisen, which could be more virulent," she said.

However, researchers admit they do not know when this could happen.

Dr Oliver Pybus, an evolutionary biologist at the University of Oxford, said: "Predicting how the virus will mutate is rather like trying to predict the weather in several months' time - virtually impossible.

"The mutation rate of the flu virus is a million times faster than in animals, so it can change very quickly. London will have already developed its own strain of swine flu, even though at the moment it may not act any differently from the rest of the UK strains.

"However, the worry is that it could change, or that should a new, more dangerous version appear anywhere in the world, it will hit London firstbecause of the number of travellers passing through."

The warning comes as new research shows swine flu penetrates deeper into the lungs and can inflict more damage than ordinary seasonal flu.

Tests on animals showed that swine flu thrives all over the respiratory system, including the lungs, and causes lesions instead of staying in the head, as seasonal flu does.

Experts also believe swine flu is closely related to the 1918 pandemic strain, which killed up to 40 million. Blood tests show many survivors seem to have immunity to swine flu.

Commenting on the research, Professor Ian Jones from the University of Reading said: "It shows that the new virus is about five times more pathogenic than seasonal H1N1 but that, nonetheless, the major outcome is recovery."

Experts have predicted half the UK population could contract swine flu this year and 150,000 could die — based on figures which suggest theH1N1 virus kills about 0.5 per cent of humans who catch it.

Virologist Nigel Dimmock, emeritus professor at the Department of Biological Sciences at Warwick University, said millions would fall ill.

He said: "It would be reasonable to assume that half the population will get flu this year. If that's all swine flu, then that would be a hugely significant number of deaths."

"The percentage of people killed who contract the virus is believed to be around 0.5 per cent. If half the population catch swine flu then that could mean 30 million times that percentage as a number of deaths — it's an awful lot of people.

"Things are certainly not going to get any better by the end of the year. At the minute everyone is susceptible."

The news comes after the recent deaths of the first victims of swine flu who were otherwise healthy.

Schoolgirl Chloe Buckley, six, from West Drayton, died on Thursday. She is believed to be the first child in Britain without underlying health problems to be killed by the virus, whileDr Michael Day, 64, from Bedfordshire, was taken to hospital on Saturday and died the same day.

Click below to see the head's letter to parents with a picture of Chloe

An unnamed patient from Essex was the other victim. The co-ordinated national response to the virus is now in the "treatment" phase, as opposed to "containment", meaning experts believe the spread of the virus is unstoppable.

Health chiefs are now working to implement a plan to vaccinate at least 30 million people by the end of the year to minimise disruption to public services and industry with people takingtime off work.